Depending on the type of vibration, different measurements methods have to be used. The two main classes of vibration measurements are time domain and frequency domain.
Time domain analysis starts by analyzing the signal as a function of time. An oscilloscope, data acquisition device, or signal analyzer can be used to acquire the signal. The plot of vibration versus time provides information that helps characterize the behavior of the structure. If behavior can be characterized by measuring the maximum vibration (or peak) level, or finding the period, (time between zero crossings), or estimating the decay rate (the amount of time for the envelope to decay to near zero). These parameters are the typical results of time domain analysis.
Time domain measurements are useful for the analysis of transient vibration phenomenon and for long term vibration inspection. Transient vibration problems are often related to impacts and other non deterministic impulse type sources. As a result of the impact the vibration amplitude strongly depends on the time, e.g. the decay process until the mechanical system comes to rest again. The long term vibration inspection is used to investigate the presence of vibration sources over a specified time period. The time period varies between several minutes up to days or weeks. The target of long term inspection is to predict the presence or absence of vibration sources in order to optimize the use of sensitive equipment.
Frequency analysis also provides valuable information about structural vibration. Any time history signal can be transformed into the frequency domain. The most common mathematical technique for transforming time signals into the frequency domain is called the Fourier Transform. The math is complex, but today's signal analyzers race through it automatically, in real-time. Fourier Transform theory says that any periodic signal can be represented by a series of pure sine tones. In structural analysis, usually time waveforms are measured and their Fourier Transforms computed. The Fast Fourier Transform (FFT) is a computationally optimized version of the Fourier Transform.
Frequency domain measurements are useful for steady state vibration. To obtain the frequency domain the measured time signal (displacement, velocity, acceleration) has to be transformed by the Fast Fourier Transformation (FFT). The resulting data shows the vibration amplitude with respect to the frequency.